Quarterly report pursuant to Section 13 or 15(d)

Earnings Per Share - Schedule of Basic and Diluted Earning Per Shares (Detail)

v3.10.0.1
Earnings Per Share - Schedule of Basic and Diluted Earning Per Shares (Detail) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Numerator        
Net income attributable to Global Brass and Copper Holdings, Inc. $ 14.8 $ 12.3 $ 51.6 $ 45.5
Denominator        
Weighted-average common shares outstanding (in shares) 22.1 21.8 22.0 21.7
Effect of potentially dilutive securities:        
Stock options and nonvested share awards (in shares) 0.3 0.3 0.3 0.4
Weighted-average common shares outstanding, assuming dilution (in shares) 22.4 22.1 22.3 22.1
Net income attributable to Global Brass and Copper Holdings, Inc. per common share:        
Basic (in usd per share) $ 0.67 $ 0.56 $ 2.35 $ 2.10
Diluted (in usd per share) $ 0.66 $ 0.56 $ 2.31 $ 2.06
Stock Repurchase Program, Authorized Amount $ 35.0   $ 35.0  
Stock Repurchase Program Expiration Date Sep. 30, 2020      
Stock Repurchase Program, Number of Shares Repurchased During Period 0.0      
Stock Compensation Plan [Member]        
Effect of potentially dilutive securities:        
Anti-dilutive shares excluded from above (in shares) 0.1 0.2 0.1 0.2